The present invention relates to methods for producing C4-C6 dicarboxylates, and more particularly, to a simple and efficient method for recovering and producing C4-C6 dicarboxylates from an alkaline waste solution generated in a caprolactam preparation process.
To prepare cyclohexanol and cyclohexanone by oxidizing cyclohexane with air in liquid phase plays an important role in fiber industry such as synthesis of nylon 6 and 66 etc. For example, caprolactam used as raw material of nylon 6 and adipic acid used as raw material of nylon 66, are both made from cyclohexanol and cyclohexanone.
In a process for preparing caprolactam, normally, cyclohexane is introduced with air and oxidized to generate cyclohexanol and cyclohexanone at a temperature 150-160xc2x0 C. and under a pressure 8-10 kg/cm2 in the presence of Co or Cr used as a catalyst. The generated cyclohexanol and cyclohexanone are further treated with subsequent processes such as oximation and Beckmann rearrangement, so as to synthesize caprolactam.
During cyclohexane oxidation, part of cyclohexane may be over-oxidized to produce some neutral substances and acidic substances that would react with neutral alcohols to form esters. Accordingly, besides main products i.e. cyclohexanol and cyclohexanone, by-products such as monocarboxylic acids, dicarboxylic acids (mainly including succinic acid, glutaric acid and adipic acid), oxycarboxylic acids, a small amount of alcohols, aldehyde, low molecular weight esters, esters containing cyclohexanol group, ketones and other unknown organic substances, also co-exist with the main products in the oxidation reaction mixture. To isolate these by-products from cyclohexanol and cyclohexanone, is usually done by saponifying and salinizing the by-products with an aqueous solution of sodium hydroxide to form an aqueous solution of organic acid salts of sodium, which is called an alkaline waste solution.
Salts of dicarboxylic acids (such as succinic acid, glutaric acid, adipic acid, etc) contained in the alkaline waste solution, are acidified and separated to form various dicarboxylic acids of succinic acid, glutaric acid and adipic acid, which dicarboxylic acids can be further esterified with methanol or other alcohols to synthesis dicarboxylates of great applicability in industry. For example, C4-C6 dicarboxylic acids are esterified with methanol to form a mixture of dimethyl dicarboxylates, which is a non-toxic environmental-friendly organic solvent with high efficiency, high burning point and great solubility solvency, and thus may be potentially commercialized in the market. Besides, dimethyl adipate can be converted by esterification/hydrogenation into 1,6-hexanediol, which is an important raw material of polyurethane resin and polyester resin, and thereby endowed with high commercial and economic value.
Conventionally, the alkaline waste solution is directly burnt and converted into sodium carbonate, so as to recover sodium contained in the alkaline waste solution. Although this treatment is simply direct, highly corrosive alkaline substances are usually produced in combustion, thereby undesirably damaging the equipment lifetime with safety concern. Also, carbon dioxide produced from combustion brings about environmental problems such as green house effect and air pollution, etc. Further, it is a significant loss of economic benefits if not capable of recovering valuable substances from the alkaline waste solution. In view of the above, associated patents disclosing recovery of valuable substances from an alkaline waste solution, are exemplified as follows.
In U.S. Pat. No. 6,063,958, an alkaline waste solution produced from a caprolactam preparation process, is firstly neutralized with a proton-containing aqueous solution of inorganic acids, for adjusting its pH value to less than or equal to 3 and separating the alkaline waste solution into an organic phase and an aqueous phase. The aqueous phase is an aqueous solution of inorganic acid salts e.g. sodium sulfate etc. The organic phase is extracted for adipic acid and 6-hydroxycaproic acid therefrom by using a proton-containing aqueous solution of inorganic acids, allowing the organic phase to be separated into layers, an aqueous layer of which is further extracted for adipic acid and 6-hydroxycaproic acid by using alcohols, ketones, esters or a mixture of any two thereof. The above-obtained extract is then adopted to extract the aqueous phase containing inorganic acid salts e.g. sodium sulfate, for retrieving adipic acid and 6-hydroxycaproic acid. As a result, the final extract is alcohols, ketones, esters or the mixture of any two thereof, with rich content of adipic acid and 6-hydroxycaproic acid. By distilling this final extract, valuable substances e.g. adipic acid and 6-hydroxycaproic acid are recovered at a yield of 50-55%.
Japan Patent Publication Sho 53-33567 discloses addition of sodium hydroxide to a reaction solution of cyclohexane oxidation. The obtained alkaline organic solution is then neutralized and adjusted its pH value with sulfuric acid, and separated into an organic phase and an aqueous phase of a sodium sulfate solution. The organic phase is firstly extracted with an inorganic solution containing sodium sulfate at concentration of 15% or more by weight. The extract is mixed with the aqueous phase, and then the mixture is extracted with an organic solvent. Such resulted extract is subsequently distilled for removing the organic solvent therefrom, so as to proceed esterification/hydrogenation for producing 1,6-hexanediol.
U.S. Pat. No. 4,442,303 discloses a method for recovering C4-C6 dicarboxylic acids from an aqueous waste solution generated in a process of adipic acid preparation. A mixture of C1-C3 alkyl alcohols and C6-C20 alkyl alcohols is mixed and esterified with the aqueous waste solution. After the reaction mixture is settled to form separate phases, an organic phase thereof is distilled to obtain C4-C6 dicarboxylic acids and esters containing C6-C20 alkyl alcohols, so as to recover the dicarboxylic acids.
U.S. Pat. No. 4,052,441 discloses that a reaction mixture obtained by catalyzing cyclohexane with air, is added with an alkaline solution to separate out an alkaline waste solution containing monocarboxylic acids, 6-hydroxycaproic acids and dicarboxylic acids. The alkaline waste solution is neutralized with sulfuric acid to separate into an organic phase of organic acids and an aqueous phase of a sodium sulfate solution. The organic phase is distilled under vacuum for removing monocarboxylic acids with low boiling point and water, and then cooled down to recover adipic acid by crystallization. After crystallization, the parent liquor is treated with two-step distillation for respectively recovering monocarboxylic acids, 6-hydroxycaproic acid and dicarboxylic acids, which can be esterified and fractionated to get ester products. The crude adipic acid obtained from crystallization can be further purified by recrystallization or esterification.
U.S. Pat. Nos. 4,271,315 and 4,316,775 disclose a method for recovering a waste solution generated in a process of adipic acid preparation. First, the waste solution is concentrated to remove part of water and volatile substances. Then, the concentrate is esterified with methanol, and subsequently extracted for extracting C4-C6 dimethyl dicarboxylates by using an insoluble organic solvent. After settling down, an organic phase is distilled for recovering the organic solvent, so as to obtain a mixture of C4-C6 dimethyl dicarboxylates.
Though the above patents provide useful methods or treatments, there are still some drawbacks in respect of recovering valuable substances as follows.
1. Extraction of valuable substances is performed by using soluble or insoluble solvents as extract agents, which extraction process is complex in proceeding, and low in total recovery yield of the valuable substances(below 50%), wherein the recovered valuable substances still contain a lot of impurities e.g. organic residues that need to be burnt or treated for removal, so that only part of the valuable substances are recovered.
2. Dicarboxylic acids recovered by crystallization mainly include adipic acid, which is low in recovery yield and purity, and thereby needs to be treated with multiple recrystallization for achieving desirable purity; whereas other valuable substances such as 6-hydroxycaproic acid and other dicarboxylic acids are unable to be recovered.
3. Direct esterification of a waste solution with alcohols can only recover some dicarboxylic acids. For example, 6-hydroxycaproic acids having hydroxyl group only form monoesters that cannot be combined with dicarboxylates for sale.
Besides adipic acid and 6-hydroxyccaproic acid, the waste solution also contains 20-40% low molecular weight ester compounds, e.g. valuable substances such as incompletely saponified esters or ketones containing cyclohexanol group and some C4-C6 lactones. If these compounds are not oxidized and converted into useful dicarboxylic acids, it results in accumulation of organic residues and decrease in a recovered amount of dicarboxylic acids. Thus, a method for recovering a waste solution generated in a process of adipic acid preparation, is not applicable to recovery of valuable substances from an alkaline waste solution produced in a caprolactam preparation process.
In response to the above drawbacks, the present inventor has researched to find out a simple and efficient method for recovering valuable substances from an alkaline waste solution so as to produce dicarboxylates, in which highly efficient oxidation and improved concentration are adopted to efficiently recover most valuable substances present in the alkaline waste solution, thereby significantly increasing economic benefits in recovery.
Therefore, a primary objective of the present invention is to provide a method for recovering and producing C4-C6 dicarboxylates from an alkaline waste solution generated in a caprolactam preparation process, which can efficiently recover most valuable substances present in the alkaline waste solution.
In a process of caprolactam production, crude products obtained from cyclohexane oxidation are saponified and salinized, and extracted with water to separate into two phases including an organic phase and an aqueous phase. The organic phase is a mixture of cyclohexanol and cyclohexanone, and the aqueous layer is so-called alkaline waste solution sodium containing sodium salt of organic acids. The alkaline waste solution is neutralized and adjusted for its pH value with sulfuric acid, so as to separate into an aqueous layer and an organic layer. The aqueous layer mainly comprising sodium sulfate, is delivered to a manufactory for recovering sodium sulfate. The organic layer contains valuable substances approximately including: formic acid 1-3%, acetic acid 1-3%, butyric acid 2-5%, valeric acid 0.1-0.5%, caproic acid 2-6%, succinic acid 0.05-0.3%, glutaric acid 0.5-1.5%, adiptic acid 8-15%, 6-hydroxycaproic acid 10-20%, water 20-30%, and other compounds 20-40% e.g. low molecular weight ester compounds such as incompletely saponified esters or ketones containing cyclohexanol group, some C4-C6 lactones, and unknown organic substances. This organic layer is therefore where valuable substances are to be recovered by using the recovery method of the invention.
The method for recovering and producing C4-C6 dicarboxylates from an alkaline waste solution generated in a caprolactam preparation process of the invention, comprises the following steps.
(1) Oxidation and conversion of valuable substances present in an organic phase:
First, neutralize an alkaline waste solution generated by oxidizing cyclohexane in a caprolactam preparation process, with sulfuric acid to adjusted its pH value and separate it into an aqueous phase and an organic phase. Add nitric acid as an oxidant to the organic phase, and proceed oxidation and conversion at suitable reaction stages under suitable temperature and pressure, so as to oxidize and convert most valuable substances present in the organic into dicarboxylic acids.
(2) Two-stage concentration of the reaction mixture after oxidation and conversion:
Introduce the reaction mixture obtained in the step (1) by oxidation and conversion with nitric acid into a two-stage concentration apparatus. The first-stage concentration is to distill out low boiling-point monocarboxylic acids and most nitric acid. In the second-stage concentration, remaining nitric acid and nitrocompounds are decomposed at higher temperature to recover most nitric acid, and thus crude concentrates mainly comprising C4-C6 dicarboxylic acids are obtained.
(3) Esterification of crude concentrates of dicarboxylic acids:
Place the crude concentrates of C4-C6 dicarboxylic acids obtained in the step (2) into an esterification apparatus, and add C1-C4 alkyl alcohols to proceed esterification with or without a catalyst. In order to desirably achieve sufficient esterification, two-stage esterification is performed. The first-stage esterification is conducted at lower temperature and pressure to get intermediates with better fluidity, which intermediates are produced by half of carboxylic acid groups being esterified. Subsequently, the second-stage esterification is performed at higher temperature and pressure, allowing esterification to be sufficiently proceeded, so as to obtain highly esterified crude dicarboxylates.
(4) Distillation of dicarboxylates:
Introduce the crude dicarboxylates obtained in the step (3) into at least one set of fractionation or distillation tower, so as to produce single-species dicarboxylates or a mixture of at least two species of dicarboxylates.